The oligonucleoside boranophosphates (BH3--ODN) are a new class of phosphorus modified oligonucleotides that differ chemically from natural DNA in that a borane (BH-) group replaces one of the non-bridging oxygens in the phosphodiester backbone. BH3--ODN are more lipophilic and much more resistant to nucleases than natural DNA; they form Watson-Crick duplexes, and activate the Rnase H-mediated cleavage of complementary RNA. The boranophosphates join the natural phosphodiester, the (mono-and di-) thioates, and the more-nuclease-susceptible 2'-F-arabinonucleic acids, as the only classes of oligonucleotides that by themselves can orchestrate mRNA degradation in RNA/oligonucleotide hybrids via the enzyme RNase H. Despite recent improvements in the utility of antisense oligonucleotides in vivo, further progress is still needed. The goals of the proposed work are: (1) To synthesize a set of borano phosphate oligonucleotides and mixed-backbone chimeras targeted against the ras gene; (2) To evaluate the potential of BH3--ODN and chimeras to activate Tnase H1; (3) To evaluate biophysical properties of BH3--ODN including the thermodynamics, structure, and kinetics of their hybridization with RNA oligonucleotides and to study the kinetics of hydrolysis of RNA in BH3--ODN /RNA/Rnase H complexes. (4) To examine penetration of BH3-ODN and localization in cells, and other pharmacological properties; and (5) to probe the sequence and structural requirements for Rnase H specificity through our chemistry. The boranophosphates provide a new platform for probing the subtle effects of structure and sequence on RNase H catalysis. Our overall goals are to better understand the unique chemical and biological properties of boranophosphate oligonucleotides and gain further insight into the mechanism of action of RNase H and antisense agents. This information should allow us to design more potent antisense drugs for treating cancer and viral diseases.